Reference Electrodes and Standard Potentials

1
Reference Electrodes and Standard Potentials The
standard hydrogen electrode (SHE) serves as the
standard reference electrode against which other
electrode potentials are referenced
Oxidation is occurring in the SHE half-cell and
this electrode is the anode H2
(1 bar) ? 2 H (aH 1) 2 e- eoH2 / H,
oxd eoH / H2 , red ? 0 V
2
Reduction is occurring in the other half-cell
which is acting as the anode 2 e- 2 H (aH
1) NO3- (aNO3- 1) ? NO2- (aNO2- 1) H2O
(l) eo NO3- / NO2- , red ? The net cell
reaction is the sum of the oxidation and
reduction ½ cell reactions H2 (1 bar) ? 2 H
(aH 1) 2 e- eoH2 / H, oxd 0 V 2 e-
2 H (aH 1) NO3- (aNO3- 1) ? NO2- (aNO2-
1) H2O (l)
eo NO3- / NO2- , red ? H2 (1 bar) NO3-
(aNO3- 1) ? NO2- (aNO2- 1) H2O (l) eo net
cell reaction 0.94 V eooxd eored
0 V eored eo NO3- / NO2- , red
0.94 V In this manner standard reduction
potentials are developed and tabulated. The WEB
site WebElements (http//www.webelements.com/web
elements/elements/text/periodic-table/redn.html)
provides an extensive on-line tabluation of
standard reduction potentials.
3
Standard hydrogen electrodes are inconvenient to
use in practice and so other stable more
convenient electrodes have been developed, e.g.,
the Saturated Calomel Electrode
When coupled to the standard hydrogen electrode
the saturated calomel electrode will act as the
cathode and the reduction reaction that will
occur is 2 e- Hg2Cl2 (s) ? 2 Hg (l) 2
Cl - (sat) What is the standard reduction
potential for a saturated calomel electrode?
eo Hg2Cl2 (s) / Hg
(l) , red
4
The selenite, SeO32 -, selenium, Se, oxidation
reduction couple when measured relative to a
saturated calomel electrode acting as the cathode
gave a potential of 0.60 V. What is the
standard reduction potential for the selenite,
SeO32 -, selenium, Se, couple?